Handbook for Methane Control in Mining - AMMSA

17THE IMPORTANCE OF HIGHER AIR VELOCITY IN PREVENTINGMETHANE EXPLOSIONSLow air velocities can lead to poor mixing between methaneand air. This poor mixing in turn leads to fluctuations in themethane concentration that make an ignition more likely.Bakke et al. [1967] first suggested that a measurement of the methane concentration alone is anincomplete means of assessing the ignition hazard and that other measurable ventilation quantitiesmight be important. 31 A study of methane ignitions in U.K. coal mines found that theprobability of an ignition is determined by both the methane concentration and the densimetricFroude number, a dimensionless quantity related to the gas-mixing process in the presence ofbuoyancy forces. The expression for the Froude number F is—2uF =∆ρg Aρ∆ρwhere u is the air velocity, is the density difference between air and methane divided by theρdensity of air, and A is the cross-sectional area of the airway.The data available to Bakke et al. resulted from 123 ignitions on faces and gate roads at U.K.longwalls during 1958–1965. Examination of the data indicated that the risk of an ignition wasdependent on more than methane concentration alone and that it was possible to combine concentrationand Froude number in one variable of the form c 2 /F.Figure 1–9 shows the normalized number of ignitions P (ignitions per year per gate road) versusc 2 /F for the Bakke et al. data. The best fit to the data was P = 0.004 (c 2 /F) 0.9 . A high correlationwas obtained, indicating that, absent other sources of mixing, the risk of ignition P does dependon the variable c 2 /F.In most mines, A does not change much compared to changes in c 2 and u 2 . Also, the factor of0.9 is close to 1.0. It follows that ignition risk varies with the quantity (c/u) 2 . This departs fromany notion that ignition risk depends on the concentration c alone. 3231 Actually, since ignition risk also depends on human factors, there is no reason to expect that ignition risk dependsonly on concentration. Mines with less gas may also have a less vigilant workforce. However, Bakke et al. onlysought a correlation with measurable ventilation quantities.32 Subsequent work at longwall shearers in the 1980s failed to confirm this finding [Creedy and Phillips 1997; CEC1985], probably because water sprays on the shearer provided enough mixing between methane and air to overcomeany velocity effect on mixing.